Dynamic Search for Responsive HTML5 - can you return results on-the-fly?

I'm producing a help in Responsive HTML5, and I was wondering if the search facility can return page results as users type, instead of having to press 'Enter' or click the magnifying glass icon.

...or perhaps that's a really bad idea. Would that cause a lot of 'processing overhead' on the site because of the multitude of calls i.e. a call for every letter typed?